I'm not sure as I haven't tested using Splunk with IE8, but assuming you downloaded the latest version of Splunk, which is 6.3.2, IE8 is not supported. The last version to support IE8 was 6.0.x.
http://docs.splunk.com/Documentation/Splunk/6.3.2/Installation/Systemrequirements#Supported_browsers
If IE8 is a requirement for you then you can find older releases of Splunk here: http://www.splunk.com/page/previous_releases
I would ensure compatibility between the version of Splunk you are using and the version of your browser first, before troubleshooting further.
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